Writing a DSL in Lua

使用Lua实现DSL。

作者：leafo

DSLs, or domain specific languages, are programming languages that are designed to implement a set of features specific to a particular problem or field. An example could be Make, the build tool, which is a specially designed language for combining commands and files while managing dependencies.

A lot of modern programming languages have so much flexibility in their syntax that it’s possible to build libraries that expose their own mini-languages within the host language. The definition of DSL has broadened to include these kinds of libraries.

很多现代编程语言，语法非常的灵活，用库的形式，在宿主语言中构他们自己的迷你语言。 用DSL扩展自己的库。

In this guide we'll build a DSL for generating HTML. It looks like this:

Dropping the parenthesis One of the cases for Lua as described in its initial public release(1996) is that it makes a good configuration language. That’s still true to this day, and Lua is friendly to building DSLs.

1996年的lua发行版的描述中就去掉了括号，使他成为一个出色配置语言，延续至今，LUA是友好的DSLs构建语言。

A unique part about Lua’s syntax is parenthesis are optional in some scenarios when calling functions. Terseness is important when building a DSL, and removing superfluous characters is a good way to do that.

唯一lua语法用到括号的场合是函数调用。构建DSL的关键点就是简洁，剔除多余字符也是很好的途径。

When calling a function that has a single argument of either a table literal or a string literal, the parenthesis are optional.

This syntax has very high precedence, the same as if you were using parenthesis:

这是个高优先级的语法， 类似于用括号。

tonumber"1234"+5-- > tonumber("1234") + 5

ChainingParenthesis-less invocation can be chained as long as each expression from the left evaluates to a function (or a callable table). Here’s some example syntax for a hypothetical web routing framework:

If it’s not immediately obvious what’s going on, writing the parenthesis in will clear things up. The precedence of the parenthesis-less invocation goes from left to right, so the above is equivalent to:

如果还是不能立马说明问题，写一个括号就一目了然了。
–>符的调用优先级是从左到右的，与上面的效果一样。

match("/post-comment")({...})

The pattern we would use to implement this syntax would look something like this:

这种模式可以实现这个语法，类似于下面这种：

localfunctionmatch(path)print("match:",path)returnfunction(params)print("params:",params)-- both path and params are now availble for use hereendend

Using a recursive function constructor it’s possible to make chaining work for any length.

使用递归函数构造,可以是让链变为任意长度。

Using function environments When interacting with a Lua module you regularly have to bring any functions or values into scope using require. When working with a DSL, it’s nice to have all the functionality available without having to manually load anything.

使用函数环境，与lua模块进行交互，你可以require任意一个变量或是函数到作用域。

One option would be to make all the functions and values global variables, but it’s not recommended as it might interfere with other libraries.

可以让全局变量让所有函数都可见，但不推荐这样，可能会影响其它的库。

A function environment can be used to change how a function resolves global variable references within its scope. This can be used to automatically expose a DSL’s functionality without polluting the regular global scope.

函数环境可以用来改变，解决函数作用域范围内的全局变量的引用。

For the sake of this guide I'll assume that setfenv exists in the version of Lua we're using. If you're using 5.2 or above you'll need to provide you own implementation:

这篇我会假设，setfenv已经存在的lua版本中使用。如果你使用的是5.2或是更高版本，你需要自己动手。

Implementing setfenv in Lua 5.2, 5.3, and above Here’s a function run_with_env that runs another function with a particular environment.

In this trivial example the benefits might not be obvious, but typically your DSL would be implemented in another module, and each place you invoke it is not necessary to bring each function into scope manually, but rather activate the whole sscope with run_with_env.

Function environments also let you dynamically generate methods on the fly. Using the __index metamethod implemented as a function, any value can be programmatically created. This is how the HTML builder DSL will be created.

函数环境也让你飞速动态的生成方法。使用__index元方法实现一个函数，任何的变量都可以自动化创建。这是HMTL创建器DSL如何被创建。

Implementing the HTML builder Our goal is to make the following syntax work:

The build_tag function is where all actual work is done. It takes the name of the tag, and the attributes and content as a single table.

build_tag函数是在所有实际工作完成时。它取得标签的名字，属性和正文是一个单个的table。

This function could be optimized by caching the generated functions in the environment table.

这函数是缓冲区优化的，生成的函数在环境table里。

The void elements, as mentioned above, are defined as a simple set:

空元素， 如上所述，定义一个简单的设置：

localvoid_tags={img=true,-- etc...}

The most efficient way to concatenate strings in regular Lua is to accumulate them into a table then call table.concat. Many calls to table.insert could be used to append to this buffer table, but I prefer the following function to allow multiple values to be appended at once:

The opts argument can either be a string literal or a table. When it’s a table it takes advantage of the fact that Lua tables are both hash tables and arrays at the same time. The hash table portion holds the attributes of the HTML element, and the array portion holds the contents of the element.